Treatment of Data

Scattering Law (S)

The calculation of the scattering law is based on the defining equation, given in the introduction.

Generalised Density of States (G)

An effective vibrational density of states can be defined as: . We take the function B(Q,w) of Price and Carpenter (J. Non-Cryst. Solids 92, 153, 1987) , where we assume the system contains only one scattering atom of average mass M. It is generally thought that an average of this function over a sufficiently large Q-range results in a fair approximation to the real GDOS.

Raw Data in Time (R)

No data manipulations are performed, and the data is displayed as total neutron counts per time channel.

Raw Data in Energy (E)

As for (R) but the time dimension is converted to energy, using .

Muphocor (M)

A Muphocor file is generated, for use in multiple scattering program of the same name.

Create Vanadium Calibration File (V)

A file of relative detector efficiencies is created. You need to use this option with the measured vanadium spectrum if you want the experimental sample data to be normalised.

The total number of neutron counts per detector is calculated at a given constant energy cut. With the fact that vanadium scatters incoherently (i.e. equally over all angles), the relative efficiencies of the detectors  are directly proportional to this total number of counts. The highest angle detector is arbitrarily assigned as the detector to normalise against.

N.B.

All transformations that include the conversion of time to energy, have an additional detector efficiency weighting since the probability of neutron capture in the helium gas detectors is related to the neutron energy.

Elastic Channel Choice

Frequent (F)

The average channel over all detectors having the maximum number of counts is determined. For each detector the channel with maximum counts is compared to its neighbouring channels to remove the possibility of counting a glitch.

Calculate (C)

There is an error in the code here. We suggest you do not use this option for the time being.

Select (S)

The elastic channel is selected manually by the user.

List (L)

A listing of detector number and the channel having maximum counts is output to the screen. The elastic channel is then selected manually by the user.

Background Treatment

Cadmium (C)

The 'cadmium' data are normalised by the monitor counts and then directly subtracted from the sample data.
An option to neglect statistical errors from the 'blank' spectrum is offered in more parameters.

Blank (B)

The 'blank' data are normalised by the monitor counts and then directly subtracted from the sample data.
An option to neglect statistical errors from the 'blank' spectrum is offered in more parameters.

Range (R)

The average number of counts over a given detector range will be subtracted from the data .

Plot (P)

You get to see the data and then decide on a detector range which you think is suitable for the subtraction.

Auto (A)

The counts are summed over detectors and a 10-channel 'window' is then passed through the data, the lowest average value is chosen as the background level.

Constant (K)

A constant background value will be subtracted after input by the user.

None (N)

No background is subtracted.

More Parameters

Sample Temperature

You can adjust this or take the default from the data file header (K).

Maximum Neutron Energy Gain

Again, you can adjust this or take the default 8kT (meV) value.

Molecular Weight 

The relative molecular mass of sample (g/mol).

Number of Moles

To do a proper scaling of the scattering power we need to know how many scattering atoms are in the beam, hence we give the number of moles of sample.

Debye-Waller Factor

Due to the Debye-Waller factor, the scattering is somewhat weaker at large Q's. The desired parameter is W=(B/16pi²), where B represents the crystallographic overall isotropic temperature factor.

Detailed Balance

This accounts for the probability of a transition involving neutron energy gain or loss being related to .

Scattering Cross-section

To correctly calculate the generalised phonon density of states, you need to give the total scattering cross-section (barns) of the sample.

Omit Error due to Background Subtraction

This only works if you are subtracting a measured background spectrum from the data.

Wrap Round Channel 1

In principle, the spectra should be continuous and the last time channel leads on to the first of the next frame. The use of this option depends on where the elastic line is located and is often unnecessary.

Compress Data

When converting to energy, the data bunched together at small energy transfers. You can reduce the number of data points, and increase the separation between them, by binning the data.

Delta Energy Separation

If you compress, this value gives the smallest energy spacing of the data.